The present invention relates to a diversity system used in land mobile radio for reducing the influence of multipath fading, thereby improving the digital transmission performance.
The interest in digital transmission in FM land mobile radio has been recently increasing. In land mobile radio, digital transmission between base station and mobile station is usually performed via multiple random paths, because of complex reflections by buildings. Thus, in case of UHF land mobile radio, rapid and deep multipath fading phenomenon will occur on the received signal as the vehicle moves. Under such multipath fading environment, digital transmission performance is greatly degraded. In order to obtain an average error rate equivalent to the digital transmission performance in an environment free of fading, the transmitter power must be considerably increased as compared with the case of no fading. However, in a land mobile radio system with such a high power, it is extremely difficult to design a transmitter which is simple in construction and inexpensive to manufacture.
In order to solve the above problems, a multi-carrier diversity system has been proposed wherein employing different carrier frequencies separated from each other by at least twice the baseband filter bandwidth related to the bit-rate of the baseband digital signal. However, in the case of a baseband digital signal of low bit-rate and high carrier frequency in the UHF range, the ratio of the difference between adjacent carrier frequencies to the carrier frequency becomes very small, and it becomes very difficult to separate carrier frequencies from each other by at least twice the baseband filter bandwidth.